Diabetes has rapidly developed into a major global epidemic. In 2011 it was announced that 336 million people worldwide suffer from type 2 diabetes, and this number is expected to grow to around 552 million by 2030. Currently, the disease is responsible for approximately 4.6 million deaths each year (Diabetes Atlas published by the International Diabetes Federation). Such a high number of instances is creating an unsustainable financial burden to healthcare systems, with treatment circulating over USD$465 billion dollars in 2011, which is a significant 11% of total healthcare expenditure in adults.
Despite current therapies, around 40% of individuals with type 2 diabetes develop a requirement for exogenous insulin administration over time, suggesting that these insulin secretory abnormalities progress after the onset of the disease. Accordingly, there is an obvious and clear need to develop more effective therapies for the treatment of type 2 diabetes, that improve the quality and/or quantity of insulin production, to address this gap in the current treatments.
Pancreatic β-Cell Insulin Secretion
Glucose-induced insulin secretion by pancreatic β-cells is generally schematized by a ‘consensus model’ that involves the following sequence of events: acceleration of glucose metabolism; closure of ATP-sensitive potassium channels (KATP channels) in the plasma membrane; depolarization, influx of Ca2+ through voltage-dependent calcium channels and a rise in cytosolic-free Ca2+ concentration that induces exocytosis of insulin-containing granules (Henguin et al., 2009).
Insulin resistance describes the state where insulin produces an inadequate biological effect, causing decreased insulin-induced muscle and adipose glucose uptake and increased hepatic glucose production. Subjects suffering from type 2 diabetes have some depletion of β-cells and exhibit increased β-cell apoptosis (Butler et al., 2002). However, the extent of β-cell depletion is controversial and trials of bariatric surgery and intense caloric restriction demonstrate that recovery of insulin production is possible in type II diabetics.
Sulfonylurea drugs have been a popular strategy for the development of novel treatments for type 2 diabetes over the last 60 years. This class of drug works by binding directly to the KATP channel, and thus inducing the secretion of insulin from the β-cells.
Endoplasmic Reticulum (ER) Stress
The ER is a major protein-folding compartment in a eukaryotic cell and is second only to the cytosol, and is the site of synthesis for all secretory and cell surface proteins. Protein folding in the ER is more complex than protein folding in the cytosol because proteins are post-translationally modified. Folding in the ER must couple protein-synthesis pathways operating outside of the compartment with ER-assisted folding (ERAF) pathways in the lumen. Expression of a mutant version of a protein, or even some-wild-type proteins, viral infection, energy or nutrient depletion, extreme environmental conditions, or stimuli that elicit excessive calcium release from the ER lumen compromise protein-folding reactions in the ER, causing unfolded protein to accumulate and initiate a cascade of signals that are transmitted to the cytoplasm and nucleus. When the protein-folding demand on the ER exceeds the folding capacity of the ER a condition termed “ER stress” results (Malhotra et al., 2007). ER stress can also result from other conditions, namely when a protein that is translated in the cytosol is misfolded and induces the recruitment of the folding machinery, leading to a deficit of folding assistance in the ER.
The overproduction of secretory proteins has been demonstrated to generate ER stress and it is well known that pancreatic islet β-cells sustain a high rate of insulin production and secretion. ER stress occurs when protein-misfolding increases beyond threshold levels, releasing a suite of signaling and transcriptional events known collectively as the unfolded protein response (UPR). The ultimate goal of the UPR is to restore ER protein biosynthesis, but paradoxically one initial step is a suppression of translation of the secretory proteins, such as insulin in the β-cell, to relieve ER load. Accordingly, the physical response mechanism, whilst protecting the secretory cell, can have adverse physiological effects. For example, in β-cells this would be a reduction in insulin production and, hence, hyperglycemia. Both β-cell ER stress and insulin misfolding have been reported in both type 2 diabetes and autoimmune type 1 diabetes.
β-cells are long-lived cells that require a highly developed ER in order to produce extremely large amounts of insulin over a long period of time. This insulin generation requires a complex series of molecular biosynthetic events that are initiated in the ER. Pre-pro-insulin, the precursor of mature insulin, is cotranslationally translocated in the ER lumen, where the signal sequence is cleaved and proinsulin is generated. Machinery enzymes in the ER catalyze the formation of three disulfide bonds, to help form its native shape, before being trafficked to the Golgi apparatus and secretory granules. Finally, in the downstream Golgi it is further processed by endoproteases and the C-peptide is removed, generating the final product, mature insulin.
Potential contributors to β-cell ER stress in type 2 diabetes include increased insulin biosynthesis, lipotoxicity due to saturated long chain free fatty acids (FFA) such as palmitate (a side effect of obesity), increased reactive oxygen species (ROS) production in the ER due to increased disulfide bond formation and by mitochondria due to high glucose, and progressive accumulation of amyloid deposits (Cnop et al., 2012, Cunha et al., 2008).
The precise molecular mechanisms governing how insulin resistance interrelates to β-cell dysfunction are currently not well understood, with many obese individuals being highly insulin resistant without developing β-cell dysfunction and diabetes. However, insulin resistance adds to the demand for increased insulin biosynthesis in β-cells. Furthermore, the inappropriate secretion of the proinsulin precursor by β-cells, which increases as diabetes progresses, is thought to contribute to increased insulin resistance. Loss of cyclicity of insulin release from the pancreas also occurs in diabetes and contributes to increasing insulin resistance. Oxidative stress, ER stress, amyloid deposition in the pancreas, as well as both lipotoxicity and glucotoxicity are believed to play a role in progressive β-cell dysfunction, and are all caused by over-nutrition.
IL-22 Treatment
Huang et al. (U.S. 2010/0015086) have shown that injection of recombinant IL-22 (rIL-22) can reduce total serum glucose, triglyceride and insulin levels as well as body weight in obese mice. They have also shown that systemic administration of rIL-22 can improve glucose tolerance and insulin sensitivity in normal mice. Based on these observations, Huang et al. propose that rIL-22 would be useful for treating metabolic disorders including obesity, diabetes, hyperlipidemia, hyperglycemia and hyperinsulinemia. However, Huang et al. do not disclose any mechanism of action to explain how rIL-22 achieves these effects.
The present inventors observe, however, that no clinical trials have been published that support the use of systemic administration of rIL-22 for treating metabolic disorders in humans since publication of Huang et al. and that this may be due to possible off-target effects of rIL-22. For example, chronic high concentrations of IL-22, and low concentrations of the counteracting IL-22 binding protein (Dumoutier et al., 2001), have been shown to contribute to hyperplasia and tumor development in the intestine in animal models (Huber et al., 2012; Kirchberger et al., 2013; Yu et al., 2013; Jiang et al., 2013). IL-22 has also been reported to induce hepatocyte proliferation and overexpression of IL-22 in the liver has been shown to increase the rate of carcinogen-induced liver cancers (Park et al., 2011). It has also been reported that IL-22 is associated with human hepatocarcinomas, and that carcinogen-induced liver cancers are reduced in IL-22 knockout mice (Jiang et al., 2011). IL-22-producing CD8 T cells have been found associated with transplant-associated skin cancers and possibly contribute to squamous cell carcinoma growth or development in this setting (Zhang et al., 2013). As shown herein, the present inventors have also discovered that systemic administration of rIL-22 has the potential to exacerbate viral infections in humans.